The present invention relates to a ball bearing used for various types of machine tools.
In recent years, speed of main spindles in machine tools has been increased for the improvement of working efficiency and productivity and, correspondingly, the number of rotation of ball bearings for use in machine tools has also been increased.
In general, when the ball bearing rotates at a high speed, large sliding is caused at the contact point between a ball and a raceway surface by spin or gyro movement. Further, the clearance in the bearing is reduced under the effect of centrifugal force exerting on an inner ring or a ball and a contact pressure between the ball and the raceway surface increases, to increase the heat value which leads to various disadvantages such as temperature elevation or seizing and progress of excessive wearing in the ball bearing.
As a countermeasure for suppressing the disadvantages, it has been adopted to decrease the diameter of the ball or to make the material of the ball with light weight ceramics to thereby moderate the increase of the surface pressure due to the centrifugal force of the ball. Further, it has also been disclosed a method of suppressing the change of the clearance in the bearing by making the material of the inner ring with a material of lower linear expansion coefficient than the material for the outer ring (Japanese Unexamined Patent Publication No. 62-24025).
When a ball bearing applied with such a countermeasure is used for a main spindle for use in machine tools, an operation is possible up to maximum dmn value of 1,500,000 to 2,000,000 showing the limiting speed. However, only by individual countermeasures in the prior art such as decrease of the ball diameter or use of ceramic material for the ball the limit for the dmn value is up to 2,000,000, and seizing or wear due to increase of the contact pressure occurs if the value is exceeded.
Particularly, it is difficult to increase the speed under slight amount oil lubrication such as grease lubrication, oil air lubrication or oil mist lubrication used frequently in bearings for use in machine tools, in which seizing caused by breakage of the oil films and increase of the surface pressure in the bearing becomes remarkable.
As the counter measure, the effect of temperature elevation can not but be prevented by increasing the amount of lubrication oil or changing the preload system from a constant position preloading system tending to increase the contact pressure due to the temperature elevation to a constant pressure preloading system in which the contact pressure is kept substantially constant free from the effect of the temperature change but this inevitably increases the cost.
The present invention has been accomplished for overcoming such disadvantages in the prior art and it is an object thereof to provide a ball bearing of a reduced cost, generating less heat and having long life, capable of adopting a slight amount oil lubrication system and a constant position preloading system which is advantageous for reduction of the cost and capable of attaining high seizing resistance and wear resistance even at high speed rotation of dmn value of 2000,000 or higher.
The ball bearing of the present invention includes balls disposed so as to roll between an inner ring and an outer ring in which a groove of an arcuate cross section is formed in a circumferential direction to each of the surfaces of the inner ring and the outer ring in contact with the balls, wherein the arcuate groove of the inner ring is formed such that the radius of curvature thereof is within a range of 52.5% or more and 60.0% or less of the diameter of the ball, and the arcuate groove of the outer ring is formed such that the radius of curvature thereof is within a range of 50.5% or more and 53.0% or less of the diameter of the ball.
When the radius of curvature of the groove in the inner ring is defined as within a range from 52.5 to 60.0% and the radius of curvature of the groove in the outer ring is defined as within a range from 50.5 to 53.0%, the heat value in the inner ring can be decreased and the heat value in the outer ring can be made closer to that of the inner ring and, thus, the heat value in the entire ball bearing can be suppressed. Therefore, seizing of the ball bearing in a high speed rotational region can be prevented without adopting a large flow rate lubrication method or a preloading method such as constant pressure preloading.
In this case, the radius of curvature for the inner ring is preferably formed within a range of 52.5% or more and 57.0% or less of the ball diameter. This can reliably suppress the heat generation in the inner ring and the heat value in the entire ball bearing can be suppressed more reliably.
Further, the invention provides a ball bearing wherein the diameter of the ball is defined as 0.32 to 0.60 times the value for: (outer diameter of outer ringxe2x80x94inner diameter of inner ring)/2.
Assuming the ball diameter/{(outer diameter of outer ringxe2x80x94inner diameter of inner ring)/2}=xcex1, decrease of value xcex1 means that the ball diameter is decreased relative to the radial cross section of the ball bearing, and this has a close concern with the heat value in the ball bearing. When the value xcex1 is controlled within a range from 0.32 to 0.60, the heat value in the ball bearing can be suppressed. If xcex1 is less than 0.32, the ball diameter is excessively small making it necessary to decrease the wall thickness of a cage and, accordingly, the collision force and the hoop stress exerting from the ball to the cage are increased during high speed rotation, making it difficult for the design of the case capable of withstanding them. Further since the ball diameter is small, the contact area with the groove of the outer and inner rings is excessively small to increase the contact pressure and thus increase the heat value, which causes oil film breakage or the like leading to deterioration of the bearing life. On the other hand, if xcex1 exceeds 0.60, the ball diameter is excessively large making the centrifugal force exerted on the ball excessively large during high speed rotation and, particularly, surface pressure increase with respective to the outer ring groove to result in lowering of the life, as well as spin sliding increases at the contact point between the ball and the raceway groove and, accordingly, the heat value increases to reduce the bearing clearance, increase the surface pressure and result in oil film breakage to cause seizing and deteriorate the life. Accordingly, the allowable range for the diameter of the ball is defined within a range of xcex1 from 0.32 to 0.60 at the maximum in the present invention.
Then, in a more preferred embodiment, the range is further narrowed such that the ball diameter is from 0.35 xcex1 to 0.55 xcex1 and, further, the ball diameter is from 0.39 xcex1 to 0.48 xcex1.
Further, the invention to a restriction for the number of balls and provides a ball bearing of the invention wherein the number of balls satisfies a relation: 1.1xe2x89xa6{pitch circle diameterxc3x97sin (180/number of balls)}/ball diameterxe2x89xa62.0
Assuming {pitch circle diameterxc3x97sin (180/number of balls)}/ball diameter=xcex2, the value xcex2 is from 1.1 to 2.0.
Decrease of value xcex2 means that the number of balls is increased in view of the design since the ball pitch space is decreased on a pitch circle diameter of a ball bearing. If xcex2 is less than 1.1, since the number of balls increases, the circumferential width for the bar of the cage has to be narrowed and, as a result, impact from the ball increases more as the rotational speed is higher tending to break the bars of the cage. Further, increase in the number of balls increases the stirring resistance of the lubricant in the bearing to increase the heat value. On the other hand, the value xcex2 in excess of 2.0 corresponds to excessive decrease for the number of balls and, since the load on the ball increases to increase the contact pressure, the spin sliding increases to increase the heat value, which leads to oil film breakage to deteriorate the bearing life.
Accordingly in the present invention, the value xcex2 is defined within a range from 1.1 to 2.0.
Further, in the invention the value xcex2 is defined as form 1.2 to 1.8 as a more preferred embodiment.
The invention intends to make an improvement in view of the material for the inner and outer rings in order to obtain a ball bearing for use in machine tools which generates less heat and suffers from less seizing, and provides a ball bearing wherein at least one of the inner ring and the outer ring comprises a steel containing, as the weight ratio, 0.2 to 1.2% of C, 0.7 to 1.5% of Si, 0.5 to 1.5% of Mo, 0.5 to 2.5% of Cr, and the balance of Fe and inevitable impurity elements, in which the surface carbon concentration is from 0.8 to 1.3% and the surface nitrogen concentration is from 0.2 to 0.8%.
Since a large load is applied between the ball and the outer ring due to the centrifugal force exerting on the ball during a high speed rotation, the state between the ball and the outer ring approaches complete rolling contact and sliding between the ball and the inner ring increases. The heat value in the bearing during high speed rotation greatly depends on the friction caused by sliding between the ball and the inner ring. Accordingly, it can be expected to attain further lowered heat generation by the use of a material of low friction coefficient and, particularly, a material of low friction coefficient under high pressure surface and high shearing condition during high speed rotation of the ball bearing. In particular, when an inner ring having large radius of curvature for the groove is used, since the contact pressure is remarkably increased on the inner ring with large sliding, a material having a low friction characteristic under high surface pressure is advantageous.
With a view point described above, the material used in the present invention can attain low friction characteristics under high speed sliding and high surface pressure by suppressing localized adhesion by the reinforcing effect for the matrix strength at a high temperature and can realize further lowered heat generation design as the bearing and, particularly, it is extremely effective in a case of slight amount lubrication with less cooling effect such as oil-air, oil-mist lubrication or grease lubrication.
The critical meanings for the effective range of each of the ingredient elements constituting the ball bearing according to the present invention will be described later.
The invention intends to reinforce the wear resistance on the surface of the raceway groove in order to improve the bearing life further at a high speed and it provides a ball bearing wherein a hard deposition film of 0.05 to 8 xcexcm thickness is applied to the raceway surface of at least one of the inner ring and the outer ring.
This can improve the wear resistance during high speed rotation with the dmn value of 2,000,000 or more at which the sliding or the surface pressure at the contact point between the ball and the raceway surface increases. The thickness of the hard deposition film is preferably from 0.1 to 5 xcexcm and, more preferably, from 0.5 to 3 xcexcm. At a film thickness of less than 0.05 xcexcm, the substrate surface is exposed greatly to reduce the performance of the hard deposition film and, as a result, the effect of improving the sliding property and the wear resistance is decreased. On the other hand, if it is more than 8 xcexcm, internal stress of the reaction product is increased to deteriorate the adhesion with the substrate tissue.
Further, the present invention provides a ball bearing wherein the ball is made of ceramics.
Ceramics have high modulus of elasticity compared with steels and can suppress the contact area at which the ball and inner and outer rings are in contact with each other. Accordingly, it is possible to more reliably suppress the amount of heat generated from the entire ball bearing.
Further, ceramics have lower density and higher heat resistance compared with steels and they can be free from the effect of centrifugal force further as the speed goes higher. In general, the outer ring undergoes a larger effect of centrifugal force of the ball than the inner ring, tending to increase the temperature in the bearing but such tendency is moderated by the use of a ceramic ball. According to the experiment made by the present inventor (to be described later), it could be confirmed that ball bearings made of steel balls (SUJ2) cause seizing at about 1,400,000 dmn, whereas ball bearings made of ceramic ball (Si3N4) could sufficiently withstand even for 2060000 dmn (25000 rpm).